My DC have a glass of champagne with us, on special occasions.
DD1 is 16, and started drinking at parties at about 15. Not much, though, and she doesn't go to many parties, really.
I know she was drinking at the Reading festival earlier this year, as she had an opinion about the various white rums on the market!
She went to a party recently, and I knew she would be drinking, so I just suggested she make sure she had something to eat, so she wasn't drinking on an empty stomach. (Actually, she was quite tired, and came home early, anyway.)
I'm not aware that DD2 (14) or DS (11) have ever had alcohol outside the home, although, of course it's possible.
I would not buy alcohol specifically for any of them, but I can't stop them spending their pocket money on it, really.
I just have to make sure they are well-informed, and trust them, really.